windows fastcall (x64) call convention (#314)

* initial set of work for windows fastcall (x64) call convention

- call conventions: rename `fastcall` to `windows_fastcall`
- add initial set of filetests
- ensure arguments are written after the shadow space/store (offset-wise)
  The shadow space available before the arguments (range 0..32)
  is not used as spill space yet.

* address review feedback

lib/codegen/src/ir/extfunc.rs

@@ -382,7 +382,7 @@ mod tests {
             CallConv::Fast,
             CallConv::Cold,
             CallConv::SystemV,
-            CallConv::Fastcall,
+            CallConv::WindowsFastcall,
             CallConv::Baldrdash,
         ]
         {